[R] lattice: showing panels for factor levels with no values

[Wolfram Fischer - Z/I/M]

How to show panels for factor levels of conditioning variables
which do have no values?

E.g. there are panels for "Grand Rapids" when they have values:
	data( barley )
	with( barley, dotplot(variety ~ yield | year * site, layout=c(6,2) ) )

There are no panels for "Grand Rapids"
when there are no values for "Grand Rapids":
	my.barley <- subset( barley, ! ( site == "Grand Rapids" ) )
	with( my.barley, dotplot(variety ~ yield | year * site, layout=c(6,2) ) )

But there is a level "Grand Rapids":
	levels( my.barley$site )
	[1] "Grand Rapids"    "Duluth"
	[3] "University Farm" "Morris"         
	[5] "Crookston"       "Waseca" 

Is there an option to show empty panels for "Grand Rapids" in ``my.barley''?

Thanks. Wolfram